1. Technical Field
The present invention generally relates to image sensors made in monolithic form and used in digital image acquisition devices such as cameras and photographic devices. The present invention relates to such devices, whether they are autonomous or belong to a device comprising other functions such as, for example, a cell phone or an automobile vehicle. The present invention more specifically relates to a so-called anti-eclipse system of an image sensor designed to avoid for a strong lighting to translate as black.
2. Description of the Related Art
An image sensor pixel comprises a photodiode used in reverse and having its junction capacitance discharged by a photocurrent according to a received light intensity. Obtaining the lighting level received by a pixel includes measuring the voltage across the photodiode at selected times, among which the end of a so-called image acquisition or integration period before and after which the pixel is reset by recharging of its photodiode. The decrease of the voltage across the photodiode during the acquisition period is proportional to the photocurrent having discharged its capacitance. For strong light intensities, the photocurrent may be such that the discharge of the junction capacitance during the acquisition phase adversely affects the reliability of the measurement. In particular, when the lighting increases beyond a given value, the electric signal provided by the sensor decreases down to zero. This so-called eclipse phenomenon results in that a very strongly lit area is interpreted as black (which is for example the case for the filament of an incandescent lamp).
US Patent Application No. 2006/0278809 describes an anti-eclipse system for an image sensor having its pixels formed of four transistors, that is, using a technique of charge transfer from the photodiodes to intermediary capacitive nodes of each pixel, to temporarily store the charges before reading by transfer to a column conductor common to several pixels. Such sensors are generally called 4 T sensors as opposed to so-called 3 T sensors having their pixels formed of three transistors and which comprise no intermediary capacitive node.